There is a method of narrowing the wavelength (frequency) interval of a wavelength multiplexing optical signal to improve frequency utilization efficiency for the purpose of increasing the transmission capacity of an optical fiber transmission system. However, this method has a problem that an influence of overlapping (cross talk) of optical spectra between adjacent channels is increased since the number of wavelengths of a wavelength-multiplexed optical signal is large, thereby deteriorating the transmission quality of the wavelength multiplexing optical signal.
For the above-mentioned problem, PTL 1 discloses a technique of making the wavelength of an optical signal to be output from each of a plurality of optical transmitters coincide with the wavelength of a reference wavelength optical signal, by using a transmission wavelength variable filter. The reference wavelength optical signal is one of a plurality of optical signals whose wavelength intervals are constant, which are obtained by frequency-modulating a predetermined light source with an output from a microwave oscillator. The technique described in PTL 1 makes wavelength intervals of a plurality of optical signals to be output from a plurality of optical transmitters coincide with a wavelength interval generated by the microwave oscillator.
In a technique described in PTL 1, each of a plurality of optical transmitters comprises the transmission wavelength variable filter, and each of the optical transmitters controls the transmission wavelength variable filter so that the intensity of a reference wavelength optical signal becomes a maximum. The optical transmitter then controls the wavelength of an optical signal to be output so that the wavelength of the optical signal to be output becomes a maximum at the transmission wavelength variable filter. As mentioned above, the technique described in PTL 1 indirectly makes the wavelength of an optical signal to be output from each of a plurality of optical transmitters coincide with a reference wavelength optical signal by using a transmission wavelength variable filter. PTL 2 describes a frequency control method of an optical transmitter or an optical receiver used in a wavelength multiplexing optical communication network.